1. Field of the Invention
The present invention relates to a method for measuring a straightness of a probe. More specifically, the invention relates to a method for measuring a straightness of a probe of a surface texture measuring instrument that measures a surface texture such as a profile and surface roughness of an object to be measured using the probe.
2. Description of Related Art
Roughness measuring instruments, profile measuring instruments, scanning coordinate measuring instruments and the like are known as surface texture measuring instruments for measuring a surface texture such as a profile and surface roughness of an object to be measured using a probe.
For instance, a scanning coordinate measuring instrument includes a Z-axis slider that is capable of vertical (i.e. Z-axis direction) movement and a probe attached to the Z-axis slider, the probe having a stylus (measurement piece) that is capable of minute displacement in Z-axis direction. During a scanning measurement, the stylus of the probe is brought into contact with an object to be measured and the Z-axis slider is vertically moved so that a push amount (i.e. push amount of the measurement piece in Z-axis direction) becomes constant.
Traditionally, the probe attached to the scanning measuring instrument exhibits only a minute displacement of the stylus in Z-axis direction and a constant push amount as shown in a surface texture measuring tracer disclosed in Document 1 (JP-A-64-53109). Accordingly, a straightness of a linear guide mechanism for moving the measurement piece in Z-axis direction rarely exerts great influence on a measurement accuracy.
Recently, in order to achieve a high-speed and low-measuring-force measurement, a system has been proposed in which a measuring range of a probe itself is widened and the measuring force is actively controlled, so that a high-speed and low-measuring-force scanning measurement can be conduced with a scanning probe itself. However, when such a probe having wide measurement range is used, the straightness of a linear guide mechanism for moving the stylus in Z-axis direction greatly influences on the measurement accuracy.
For instance, as shown in FIG. 7, when a stylus 14A of a probe 8 displaces in Z-axis direction by Δz, the stylus 14A deviates in X-axis direction or Y-axis direction according to the straightness of the linear guide mechanism provided in the probe 8. Then, a detected value based on the displacement in X-axis and Y-axis indicates a point different from a measured point of the stylus 14A, thereby causing a measurement error.
Thus, in order to correct a movement accuracy of the linear guide mechanism provided in the probe, the straightness is measured in a traditional measuring instrument and the like.
In a traditional straightness measurement, as shown in FIGS. 8A and 8B, an electro-capacitance sensor 101 is attached to an end of a spindle 140 in place of a stylus 14A. Then, the spindle 140 is moved in Z-axis direction while measuring gaps between the sensor 101 and a datum surface 103 (reference plane) such as a gauge block 102 at respective positions, thereby obtaining a straightness of the spindle 140 (i.e. a linear guide mechanism for moving the spindle).
However, since the electro-capacitance sensor has to be attached in place of the stylus each time the straightness is measured, a separate electro-capacitance sensor has to be prepared.
Such a separate electro-capacitance sensor accompanies a great economic burden and attachment/detachment work. Further, the accuracy for measuring the straightness is influenced by the accuracy of the electro-capacitance sensor to be used.
Furthermore, since a bending force of a cable that connects the electro-capacitance sensor and a controller is applied as an external force on the linear guide mechanism for moving the spindle, the spindle depicts a movement locus different from that depicted without the electro-capacitance sensor being attached (i.e. with the stylus being attached), thus failing to provide an accurate straightness measurement.